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The Moyal-Lie Theory of Phase Space Quantum Mechanics

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The Moyal-Lie Theory of Phase Space Quantum Mechanics The Moyal-Lie Theory of Phase Space Quantum Mechanics T. Hakio˘glu Physics Department, Bilkent University, 06533 Ankara, Turkey and Alex J. Dragt Physics Department, University of Maryland, College Park MD 20742-4111, USA Abstract A Lie algebraic approach to the unitary transformations in Weyl quantization is discussed. This approach, being formally equivalent to the ?-quantization is an extension of the classical Poisson-Lie formalism which can be used as an efficient tool in the quantum phase space transformation theory. The purpose of this paper is to show that the Weyl correspondence in the quantum phase space can be presented in a quantum Lie algebraic perspective which in some sense derives analogies from the Lie algebraic approach to classical transformations and Hamiltonian vec- tor fields. In the classical case transformations of functions f(z) of the phase space z =(p, q) are generated by the generating functions µ(z). If the generating functions are elements of a set, then this set usually contains a subsetG closed under the action the Poisson Bracket (PB), so defining an algebra. To be more precise one talks about different representations of the generators and of the phase space algebras. The second well known representation is the so called adjoint representation of the Poisson algebra of the ’s in terms of the classical Gµ Lie generators L µ over the Lie algebra. These are the Hamiltonian vector fields with the G Hamiltonians µ. By Liouville’s theorem, they characterize an incompressible and covariant phase space flow.G The quantum Lie approach can be based on a parallelism with the classical one above. In this case the unitary transformations are represented by a set of quantum generating functions µ(z) which are the quantum partners of the µ’s. The non commutativity is encoded A G ˆ in a new multiplication rule, i.e. ?-product in z. The quantum Lie generators V µ over A the Lie bracket are the adjoint representations of the generating functions µ(z)overthe Moyal bracket and they correspond to quantum counterparts of the classicalA Hamiltonian vector fields L µ . They are represented by an infinite number of phase space derivatives. G The only exception is the generators of linear canonical transformations defining the sp2(R) sub algebra. In this case the generators are given by the classical Hamiltonian vector fields generated by quadratic Hamiltonians and the classical and quantum generators are identical [see (46) below]. The classical Lie approach is sometimes referred to as the Poisson-Lie theory (PLT). This terminology is quite appropriate for generalizations as the algebraic representations that are relevant are written by using the Poisson and Lie product rules. For a similar reason we suggest that the quantum approach be referred to as the Moyal-Lie theory (MLT). In section 1 we start with a brief textbook discussion of the PLT. The Lie algebraic treat- ment of the quantum transformation theory starts in section 2 with a discussion of Weyl correspondence (section 2.1) and the ?-product. The main results of this paper, the MLT and the ?-covariance are discussed at length in section 2.2. The associativity property is discussed comparatively with respect to Poisson-Lie and the Moyal-Lie algebras in section 3. In the light of the classical covariance versus ?-covariance of the phase space trajecto- ries under canonical transformations the time evolution deserves a specific attention. The time evolution in the MLT and its extended ?-covariance property is examined in section 4. Finally, we discuss the equivalence of the MLT to the standard ?-quantization through the ?-exponentiation in section 5. A. The Poisson-Lie Theory and the classical phase space The classical Lie generators L µ are given by G T L µ =(∂z µ) J∂z , (1) G G where µ = µ(z)’s are in the set of phase space functions (generating functions) where µ =1,...,NG Gdescribes the generator index, N being the total number of generators, z denotes the 2n dimensional phase space row vector z =(z1,...,z2n)=(q1,...,qn; p1,...,pn), ∂ =(∂ ,...,∂ ), T is the transpose of a row vector, and J is the 2n 2n symplectic z z1 z2n × matrix with elements (J)ij ,i,j=(1,...,2n) 1 if j=i+n (J)ij = 1 if i=j+n . (2) 8 − < 0 all other cases The classical algebra of the phase space: generating functions is defined over the Poisson bracket (PB) , (P ) =(∂ )T J (∂ )(3) {Gµ Gν} z Gµ z Gν and that of the generators L µ over the Lie Bracket (LB) G (P ) [L µ ,L ν ] L µ L ν L ν L µ = L µ, ν (4) G G ≡ G G − G G {G G } where the LB is defined by the first equality. A classical canonical transformation (CCT) is considered to be a symplectic phase space map ~ : z Z~(z)withZ =(Q1,...,Qn; P1,...,Pn) describing the new set of canonical phaseM space pairs.7→ The symplectic condition is the invariance of the PB given by z ,z (P ) = Z ,Z (P ) = J (5) { j k} { j k} jk which implies that ~ is invertible. If the symplectic map is continuously connected to the identity within a domainM ~ = ~0, i.e. = I, Lie’s first theorem1 states that the first order M~=~0 infinitesimal change in zj is (1) δj(z, δ )=δµ L µ zj . (6) G 2 where a summation over the generating function index µ is assumed. Finite canonical transformations can be obtained by infinitely iterating Eq. (6) to an exponential operator as Zj =exp µ L µ zj (7) { G } (P ) and following [L µ ,f]= µ ,f and considering f = zj, G k {G k } 1 Z = z + ,z (P ) + , ,z (P ) (P ) + ... j j µ {Gµ j} 2! µ1 µ2 {Gµ1 {Gµ2 j} } 1 + ... , ..., ,z (P ),... (P ) + ... (8) k! µ1 µk {Gµ1 { {Gµk j} } where summations over the repeated indices are assumed. Let us consider the case N =1. This allows us to simplify the notation and drop the index µ. In fact, the generalization for finite N is nontrivial because of the possibility of additional structures in the Poisson algebra, for instance, PB of the generators µ,whereµ =1,...,N, may be closed under a specific algebra with N generators. For whatG we present here such algebraic generalizations will not be needed. It can be shown that, if f(z) has a convergent Taylor expansion in powers of z in some domain z , the transformation of f is ∈D f(Z)=exp L f(z)exp L (9) { G} {− G} where it is to be noted that Eq. (9) is an operator relation, i.e. it can be multiplied on the left and/or right by any arbitrary function g(z) without changing its validity. The transformed variable Z and f(Z) must also be well defined in the same domain . From Eq. (9) it follows the remarkable manifest covariance property2,3 D [exp L f](z) f(z)=f([exp L z]) = f(Z(z, )) , (10) { G} ≡ { G } and the generalized Leibniz rule exp L [f(z) g(z)] = [exp L f][exp L g]=f(Z) g(Z) (11) { G} { G} { G} which can be summarized in exp L [f(z) ...]=[exp L f(z)] [exp L ...]=f(Z)[exp L ...] . (12) { G} { G} { G} { G} In Eq’s (10-11) the square brackets indicate that the operator on the left acts on the object within the brackets only. In (12) the dots indicate the possibility of further functions to be acted upon. Although Eq’s (10-12) are different ways of writing Eq. (9) we will keep them explicitly for that they facilitate a comparison with their quantum analogs. The MLT is a generalization of the Poisson-Lie theory to the non commutative phase space. It will be shown that, the Moyal-Lie generators can be uniquely derived in the quantum case with similar covariance properties when the role of the ordinary functional product is taken by the ?-product. As it turns out, the manifest covariance property in Eq. (10) is modified4 because of the non commutativity of the ?-product. As opposed to the standard covariance in (10) under ordinary multiplication the new covariance rule is manifest under the ?-multiplication of the functions of the phase space. The classical covariance is then recovered in theh ¯ 0 limit of the ?-covariance. → 3 B. Quantum Lie generators The quantum mechanical results presented here are corollaries of a recent work of one the present authors4 on the extended covariance in non commutative phase space under canonical transformations5. Our formulation here is based on Weyl correspondence6–8 as a consistent and analytic principle between the classical and the quantum phase spaces. 1. The Weyl correspondence Letz ˆ =(ˆz1,...,zˆ2n)=(ˆq1,...,qˆn;ˆp1,...,pˆn) describe the phase space operators satisfying, as usual, [ˆzi, zˆj]=ihJ¯ ij and zero for all other Lie Brackets (commutators). We adopt the same definitions for z and ∂z as in the classical case. The Weyl map is a correspondence rule between the operators described by ˆ = (ˆz) and the functions f(z) denoted by ˆ f and expressed as F F F⇔ ˆ = dµ(z) f(z) ∆(ˆ z) ,f(z)=Tr ˆ ∆(ˆ z) (13) F Z {F } n where dµ(z)= k=1 [dqk dpk/(2πh¯)]. We adopt the same definitions for z and ∂z as in the ˆ classical case. HereQ ∆(z) describes a mixed basis for operators and functions in the phase space given by iωT Jz/¯h iωT J z/ˆ ¯h ∆(ˆ z)= dµ(ω) e− e (14) Z 2n where ω =(ω1,...,ω2n) is in the same domain as z, i.e.
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